The present invention relates to a device and to a method which are suitable for controlling exhaust emissions from engines using natural gas, such as CNG, LNG, etc., as a fuel.
Generally, as a means for controlling exhaust emission from the engines, a catalyst, such as a catalytic converter of rhodium, has been provided in an exhaust passage of an engine. However, such a catalyst cannot purify noxious gas when the temperature thereof does not reach an activation temperature (for example, 200xc2x0 C.), and therefore, in the case in which the temperature of the exhaust emission is lower than the activation temperature, such as in the case of engine starting, or the like, the catalyst cannot sufficiently purify the exhaust emissions.
However, solutions to such problems have been attempted by techniques as described in Japanese Unexamined Utility Publication No. 60-190923 and Japanese Unexamined Patent Publication No. 63-68713. In the former, an adsorbent is provided in an exhaust passage, and unburned gas in exhaust gas at a low temperature is adsorbed by this adsorbent. In the latter, an exhaust passage is divided into a main passage and a bypass passage at the upstream of the catalyst, a switching valve is provided at a turning-point of these passages, and an adsorbent is provided in the bypass passage in order to adsorb unburned gas in the exhaust gas. These publications teach that a flow of the exhaust gas from the engine can be appropriately switched between toward the main passage and toward the bypass passage by the switching valve, depending on a temperature of the exhaust gas, and the exhaust gas can be purified in both the case of a low temperature and in the case of a high temperature.
In addition, the adsorbent has a property in which the unburned gas is adsorbed at low temperatures, the adsorption ability thereof gradually decreases with increase in temperature, and adsorption components are desorbed, for example, when the temperature reaches 80xc2x0 C. Therefore, there is a problem in the prior techniques as described in the above publications, in that the unburned gas is discharged in the air and the exhaust gas is not sufficiently purified, in the case in which the temperature of the exhaust gas is in the intermediate temperature range from the desorption temperature of the adsorbent to the activation temperature of the catalyst.
As a technique for solving such a problem, a composition in which a sub-catalyst having a heater is provided in an exhaust passage in addition to the adsorbent and catalyst in order to forcibly heat by a heater, is described in Japanese Unexamined Patent Publication No. 6-33747. The publication says that in the case in which the exhaust gas temperature is in the above intermediate temperature range, the unburned gas can be purified by the sub-catalyst heated to the activation temperature by a heater.
However, there was a problem in that the structure thereof is made to be complicated and the number of parts for constructing it is increased, in order to form a purification device provided with a sub-catalyst having a heater as described above. Additionally, with respect to all the prior techniques described above, since the adsorbent is provided in an exhaust passage, the adsorbent is required to have quite good heat-resistance. Therefore, the cost thereof is increased, because most of such adsorbents are expensive.
Therefore, it is an object of the present invention to provide an exhaust emission control device and an exhaust emission control method, in which a complicated structure is not required, the cost is low, and exhaust gas can be sufficiently purified, in an engine using natural gas as a fuel.
An exhaust emission control device of the present invention comprises an engine using natural gas as fuel gas, a fuel tank for storing the fuel gas, a suction air passage for supplying the fuel gas from the fuel tank to the engine, an exhaust passage for discharging exhaust gas from the engine, a fuel supply port connected to the suction air passage, for supplying the fuel gas to the fuel tank, an adsorbent disposed between the fuel supply port and the fuel tank in the suction air passage, for adsorbing non-methane hydrocarbons in the fuel gas, an adsorbent heating means for heating the adsorbent, an exhaust emission control catalyst disposed in the exhaust passage, a catalyst temperature measuring means for measuring the temperature of the catalyst, and a control means for controlling the adsorbent heating means depending on the measured results obtained by the catalyst temperature measuring means.
According to this device, since the adsorbent is disposed in the suction air passage, a temperature thereof is generally maintained to be at lower than a temperature at which non-methane hydrocarbons (Non-Methane Organic Gases, hereinafter referred to as xe2x80x9cNMOGxe2x80x9d) which are noxious components in the fuel gas, can be adsorbed, that is, the desorption temperature of the NMOGs (for example, 120xc2x0 C). Here, the fuel gas passes through the adsorbent placed along the suction air passage which is a supply passage thereof, when the fuel gas is supplied from the fuel supply port to the fuel tank. Since NMOGs in the fuel gas are adsorbed on the adsorbent when the adsorbent is passed, the fuel gas stored in the fuel tank is purified by decreasing the NMOG concentration thereof. Thus, the NMOG concentration in the exhaust gas is decreased even if the temperature of the exhaust gas is at a low temperature which does not reach an activation temperature of a catalyst, since the fuel gas has already been purified when it is stored in the fuel tank. It is desirable that the adsorption capacity of the adsorbent be suitably set in comparison with the capacity of the fuel tank, so that NMOGs are reliably adsorbed by the adsorbent during the supplying of the fuel gas. As an adsorbent, specifically, activated carbon, silica gel, zeolite, etc., can be employed.
Then, because the NMOGs have been adsorbed to the adsorbent after the fuel gas is supplied, it is necessary to regenerate the adsorbent by desorbing the NMOGs and to purify the desorbed NMOGs. For that purpose, the following operation may be carried out by the control means. In the control means, the temperature of the catalyst for controlling the exhaust emission is transmitted by a catalyst temperature measuring means at all times, and when this temperature has reached the activation temperature of the catalyst, an adsorbent heating means is operated by the control means and the adsorbent is heated over a desorption temperature. Then, the NMOGs are desorbed from the adsorbent, the NMOGs reach the catalyst in an active state through the engine and are purified by the catalyst.
The device according to the present invention is characterized in that an adsorbent is disposed between a fuel tank and a fuel supply port along a suction air passage, as a first point, and in that an adsorbent heating means and a catalyst temperature measuring means are provided, and when the temperature of the catalyst has reached the activation temperature, the adsorbent is heated and NMOGs are desorbed by using a control means, as a second point. Therefore, exhaust gas from engines can be purified over the entire temperature ranges even if the engines have relatively simple structures. In addition, the adsorbent is not required to have heat-resistance because it is disposed in the suction air passage, and as a result, an inexpensive adsorbent can be used and reduction in costs can be realized.
Next, an exhaust emission control method for a natural gas engine according to the present invention relates to a process which desorbs NMOGs adsorbed to the adsorbent in the above exhaust emission control device of the present invention, and comprises of an adsorbent heating means for heating an adsorbent which is disposed in a suction air passage leading from a fuel tank for storing natural gas as fuel gas to an engine, so as to adsorb NMOGs in fuel gas, a catalyst temperature measuring means for measuring the temperature of the catalyst which is disposed in an exhaust passage for discharging exhaust gas from said engine, so as to control exhaust emission, and a control means for controlling the adsorbent heating means in accordance with the measured temperature which is supplied by the catalyst temperature measuring means, wherein the NMOGs in fuel gas flowing in the suction air passage are adsorbed by the adsorbent, the adsorbent heating means is operated by the control means, so that the adsorbent is heated over the desorption temperature at which the NMOGs are desorbed from the adsorbent, when the measured temperature in the catalyst temperature measuring means reaches the activation temperature of the catalyst.